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## Chapter summary

• Some of the scientists who have contributed to the theory of the atom include J.J. Thomson (discovery of the electron, which led to the Plum Pudding Model of the atom), Marie and Pierre Curie (work on radiation), Ernest Rutherford (discovery that positive charge is concentrated in the centre of the atom) and Niels Bohr (the arrangement of electrons around the nucleus in energy levels).

• Because of the very small mass of atoms, their mass is measured in atomic mass units (u). 1 u = 1,67 × 10−24 g.

• The relative atomic mass of an element is the average mass of all the naturally occurring isotopes of that element. The units for relative atomic mass are atomic mass units. The relative atomic mass is written under the elements' symbol on the periodic table.

• An atom is made up of a central nucleus (containing protons and neutrons), surrounded by electrons. Most of the atom is empty space.

• The atomic number (Z) is the number of protons in an atom.

• The atomic mass number (A) is the number of protons and neutrons in the nucleus of an atom.

• The standard notation that is used to write an element, is $​ZAX$, where X is the element symbol, A is the atomic mass number and Z is the atomic number.

• The isotope of a particular element is made up of atoms which have the same number of protons as the atoms in the original element, but a different number of neutrons. This means that not all atoms of an element will have the same atomic mass.

• Within each energy level, an electron may move within a particular shape of orbital. An orbital defines the space in which an electron is most likely to be found.

• The electron configuration is the arrangement of electrons in an atom, molecule or other physical structure.

• Energy diagrams such as Aufbau diagrams are used to show the electron configuration of atoms.

• The electron configuration of an atom can be given using spectroscopic notation.

• Different orbitals have different shapes: s orbitals are spherically shaped and p orbitals are dumbbell shaped.

• The electrons in the outermost energy level are called valence electrons.

• The electrons in an atom that are not valence electrons are called core electrons.

• Atoms whose outermost energy level is full, are less chemically reactive and therefore more stable, than those atoms whose outermost energy level is not full.